Environ. Sci. Technol. 2010, 44, 6517–6518
Comment on “Surface Complexation of Catechol to Metal Oxides: An ATR-FTIR, Adsorption, and Dissolution Study” Interaction of polyphenols and metals is not a new topic and has been studied for many decades (1, 2). Oxidation of polyphenols by metals have attracted a great deal of attention not only because of the complicated chemical processes involving coordination, sorption, and dissolution but also because the reaction has significant implications to various environmental (e.g., formation of soil profiles and the humic substances) and physiological processes (trigging and quenching the radical production in human body). The oxidation/reduction process usually starts with the initial complexation of the oxidative metal ion (either in solution or on solid surface) and the polyphenols (3-6) and proceeds with the electron transfer occurring in the complex (internal redox reaction) (7). Meanwhile polyphenols are also susceptible to oxidation by the air and the process could be promoted by the presence of the mineral surface in the phenol solution. The mineral surface acts as a catalyst to the oxidation process so that the absorbed phenols and oxygen can react in rather low concentrations. Sometimes, the reduction product of metals, especially Fe(II), could be reoxidized by the air further complicating the reaction if the research is conducted without strict exclusion of the air (8, 9). This paper (1) did not include much study and discussion on the oxidation of catechol by metal oxides, especially by Fe2O3 and MnO2, which surely compromised the quality of the related research. Some specific comments on the paper are as follows: 1. The reduction products of the metals were dissolved Fe(II) or Mn(II) but due to the extreme sensitivity of Fe(II) to the air (especially in higher pH) (6, 10) and low solubility of the Fe(III), failure to observe the Fe(II) usually results in the misleading conclusion that Fe(III) minerals did not oxidize the catechol. Mn(II) is relatively stable in the air and reduction of MnO2 by polyphenols to Mn(II) have been widely observed relative to the case of Fe(III) minerals. It is widely believed that release of Mn(II) from MnO2 to the solution is a redox process instead of only a simple complex (9, 11). This is also used to explain why polyphenols are able to release more Mn from the minerals or soils than Fe or Al, which could also be used to explain the highest dissolution rate of MnO2 by catechol as shown in Figure 5C. Another evidence of the major role played by redox dissolution process is that the free Mn species in Figure 5C should be Mn(II) instead of Mn(IV) because of the highly insoluble property of Mn(IV). Third, formation of the outer-sphere complex hardly seemed an explanation for the dissolution of MnO2: How does a long-range and weak interaction act more strongly on the solids than a short-range one (inner-sphere complexation as present in Fe2O3 and other two minerals)? As far as the dissolution of MnO2 is concerned, how is the outer-sphere complexation possible to compete with the strong chemical bond present inside the MnO2 lattice and dissolve the metal ion to the solution? 2. The oxidation products of catechol are not necessarily only o-quinones. The products could be quinone or semiquinone: the latter is a highly active radical and can undergo further reactions especially oligomerization and polymerization (12-15). Formation of polymerized polyphe10.1021/es102068s
2010 American Chemical Society
Published on Web 07/22/2010
nols from simple phenols has been widely reported and regarded as one of the important mechanisms for humic substances formation in the soils (16). Existing literature seems supportive that the similar oxidation reaction could have happened in each catecho-metal oxide system studied in this paper. Therefore, to support the authors’ argument, strong evidence including HPLC data, should have been provided to confirm the high recovery rate of catechol in each system besides MnO2-catechol, which was not found in the paper or the supporting documents. 3. The solubility of each metal oxide was present in Figure 5 but those data are so different from the solubility of the related metal oxides calculated using either model (Visual Minteq) or published solubility products. For example, according to Figure 5A, [Fe] ≈ 0.25 ppm for both ionic strength at pH 7, but Minteq showed [Fe] ) 2.4 × 10-9 ppm in 0.1 M KCl solution and 2.2 × 10-9 ppm in 0.01 KCl solution. Using solubility product constant of Fe(OH)3 (Ksp) 2.79 × 10-39), [Fe] ) 1.56 × 10-13 ppm present in a neutral solution. In contrast to Figure 5B, Minteq showed that dissolved Ti concentration was around 1 × 10-3 ppm and not affected by pH at pH range of 4-9. Has this been caused by the higher solubility of the nanosized particles or by failure to completely separate the nanoparticles from the solution? What was the definition of the Free Metal? How would you relate the specifically manufactured nanoparticles to the natural environmental processes? In a word, oxidation is an important reaction between Mn/Fe oxides and catechol and discussing this process in detail is necessary in the related studies.
Literature Cited (1) Gulley-Stahl, H.; Hogan, P. A., II; Schmidt, W. L.; Wall, S. J.; Buhrlage, A.; Bullen, H. A. Surface complexation of catechol to metal oxides: An ATR-FTIR, adsorption, and dissolution study. Environ. Sci. Technol. 2010, 44, 4116–4121. (2) Athavale, V. T.; Prabhu, L. H.; Vartak, D. G. Solution stability constants of some metal complexes of derivatives of catechol. J. Inorg. Nucl. Chem. 1966, 28, 1237–1249. (3) Chvatalova, K.; Slaninova, I.; Brezinova, L.; Slanina, J. Influence of dietary phenolic acids on redox status of iron: Ferrous iron autoxidation and ferric iron reduction. Food Chem. 2008, 106, 650–660. (4) Perron, N. R.; Brumaghim, J. L. A review of the antioxidant mechanisms of polyphenol compounds related to iron binding. Cell Biochem. Biophys. 2009, 53, 75–100. (5) Kennedy, J. A.; Powell, H. K. J. Aluminum (III) and iron (III) 1,2 diphenolate complexes: a potentiometric study. Aust. J. Chem. 1985, 38, 659–667. (6) Powell, H. K. J.; Taylor, M. C. Interaction of Fe(II) and Fe(III) with gallic acid and its homologues: A potentiometric and spectrophotometric study. Aust. J. Chem. 1982, 35, 739–756. (7) Hider, R. C.; Liu, Z.; Khodr, H. H. Metal chelation of polyphenols. In Flavonoids and Other Polyphenols; Packe, L., Ed.; Academic Press: New York, 2001; Vol. 335, pp 190203. (8) McBride, M. B.; Wesselink, L. Chemisorption of catechol on gibbsite, boehmite and noncrystalline alumina surfaces. Environ. Sci. Technol. 1988, 22, 703–708. (9) McBride, M. B. Adsorption and oxidation of phenolic compounds by iron and manganese oxides. Soil Sci. Soc. Am. J. 1987, 51, 1466–1472. (10) Kennedy, J. A.; Powell, H. K. J. Polyphenol interactions with aluminum(III) and iron(III): Their possible involvement in the podzolization process. Aust. J. Chem. 1985, 38, 879888. (11) Lehmann, R. G.; Cheng, H. H.; Harsh, J. B. Oxidation of phenolic acids by soil iron and manganese oxides. Soil Sci. Soc. Am. J. 1987, 51, 352–356. VOL. 44, NO. 16, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
6517
(12) Wang, T. S. C.; Li, S. W.; Ferng, Y. L. Catalytic polymerization of phenolic compounds by clay minerals. Soil Sci. 1978, 126, 21. (13) Shindo, H.; Huang, P. M. Role of Mn(IV) oxide in abiotic formation of humic substances in the environment. Nature 1982, 298, 363–365. (14) Shindo, H.; Huang, P. Catalytic effects of manganese (IV), iron(III), aluminum (III) and silicon (IV) oxides on the formation of phenolic polymers. Soil Sci. Soc. Am. J. 1984, 48, 927–934. (15) Naidja, A.; Huang, P. M.; Bollag, J. M. Comparison of the reaction products from the transformation of catechol
6518
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 44, NO. 16, 2010
catalyzed by birnessite or tyrozinase. Soil Sci. Soc. Am. J. 1998, 62, 188–195. (16) Larson, R. A.; Hufnal, J. M., Jr. Oxidative polyerization of dissolved phenols by soluble and insoluble inorganic species. Limnol. Oceanogr. 1980, 25, 505–512.
Ruiqiang Liu Department of Chemistry and Biochemistry, Miami University, Oxford, Ohio 45056 ES102068S
